Paper board is widely used throughout the world in packaging applications. Paper board can be printed and folded into attractive and functional containers that are inexpensive, protect their contents, and are based on renewable and recyclable raw materials. Paperboard's poor barrier properties limit its usefulness in food packaging, especially in applications that require high barrier resistance to liquid water, water vapor, gas permeability, oil and grease, slip, and static. To overcome this limitation, others have added additional functional layers to the paperboard, thus increasing the paperboard's barrier properties. For example, laminated films, extruded polymer coatings, and wax coatings are known to improve paperboard's resistance to both liquid water and water vapor. These coatings require additional processing, are expensive relative to the cost of the untreated paperboard, and make the paperboard harder to recycle.
Recently, however, recyclable water-based latex barrier coatings have become available that improve paperboard's barrier properties while maintaining the paperboard's recyclability. These recyclable barrier materials form a continuous film that covers the paper or paper board and gives it the required properties for demanding packaging applications. The water-based barrier coatings are generally comprised of an anionic latex and optionally a pigment. The most widely used water-based latexes are styrene butadiene latex and styrene acrylate latex. The most widely used pigments are kaolin clay, ground calcium carbonate, talc, and mica. Examples of water-based latex barrier coatings are readily available from Michelman Inc., Cincinnati, Ohio and Spectra-Kote, Gettysburg, Pa. These recyclable functional polymer coatings still require additional processing and are expensive relative to the cost of untreated paperboard.
For many demanding food packaging and other demanding applications, at least two layers of functional barrier top coating must be applied, further increasing the cost of the final product. Subsequent coatings are necessary to eliminate pinholes and to increase the overall strength and performance of the paperboard. It is well known in the industry that an inexpensive and less functional base coat may be applied to reduce both the overall porosity of the paperboard and the amount of functional top coat required. The most commonly used base coats include, but are not limited to, kaolin clay, talc, or calcined clay modified with a latex binder, such as modified styrene butadiene, styrene-acrylate, and polyurethane latexes. For example, a base coat of kaolin clay and styrene-butadiene latex requires a coating weight of between 9 to 27 g/m2 to improve the Cobb sizing of a functional top coat of Popil.
Cationic pigments are also well-known in the industry and are known to give improved properties over the same pigment in anionic form. In the industry, most cationic wet strength resin treated pigments have been treated at a resin addition level of less than 10%, based on the dry weight of the pigment. Generally, these coatings have been used as top coats. There is, however, still a need in the industry for cost-effective ways to provide a paperboard product for processes that require highly resistant barrier properties.
Water-based pigment coatings are also often added to one or both side of paper or paper board to improve the appearance of the paper or paper board, or to improve print quality. As an example, No. 5 ground-wood containing, light weight coated offset sheet is coated with a blend of kaolin/GCC/latex which provides 70% brightness, 50% of gloss, and a Parker Print Surf smoothness of 1.20. Water-based pigment coatings are generally comprised of a pigment or mixture of anionic pigments, and an anionic latex binder. The most widely used pigments are kaolin clay, ground calcium carbonate, and titanium dioxide. The most widely used synthetic binders are styrene butadiene (SB) latex and styrene acrylate (SA) latex Examples of some commonly used SB latex include Dow RAP316, Dow 620, BASF Styronal 4681 and SA latex, BASF Acronal S504. In demanding applications two to three layers of pigment coating are needed to obtain the desired appearance and print quality. There is also a need to reduce the number of coating steps and the amount of pigment coating needed to obtain the desired appearance and print quality.